New metric: Cohen's kappa (#15443)

* ENH: implement cohen_kappa

* TST: add test cases

* DOC: add document

* DOC: revise document as suggested

* CLN: drop total prefix

* CLN: wrap lines to 80 chars in metric_ops.py

* CLN: remove unneeded dependency

* DOC: support labels datatype

* TST: add explanation for testBasic

* TST: remove testBasic2

* TST: replace all random data

* CLN: remove sklearn dependency

* TST: wrap the lines in this test to 80 chars

* DOC: clean format

* CLN: typo

* CLN: lables specifiy -1 axis

* CLN: remove float lables support

* DOC: modify document of _safe_div

* CLN: rename predictions -> predictions_idx

* TST: invalid dim case

* CLN: clean codes

* DOC: remove total_ in update_op